Conventionally, curable resins such as epoxy resin are used widely in the fields of molding compound, laminate material and adhesive material. These curable resins require rapid curing from the viewpoint of improvement in productivity, and thus compounds for accelerating curing reaction, that is, curing accelerators, are generally used in curable resin compositions. Compositions based particularly on epoxy resin among curable resins are widely used in the field of encapsulation for elements of electronic parts such as transistor and IC. This is because the epoxy resin has well-balanced properties with respect to moldability, electrical property, humidity resistance, heat resistance, machine characteristic, and adhesion to an insert. Particularly, a combination of an ortho-cresol novolac type epoxy resin and a phenol novolac curing agent has excellent balance among the above-mentioned characteristics, and is thus mainly used as a base resin in a molding compound for IC encapsulation. In such epoxy resin compositions, curing accelerators for example nitrogen-containing compounds such as tertiary amine, quaternary ammonium, 1,8-diazabicyclo[5.4.0] undecene-7 (DBU) and imidazole, and phosphorus compounds such as phosphines and phosphonium salts, are generally used.
However, curable resin compositions using such curing accelerators are poor in storage stability, and thus the resin compositions should be stored and transported at low temperatures, resulting in higher costs. Because of these problems, there has been demand for development of curing accelerators making curable resin compositions excellent in storage stability.
For stabilizing storage stability, inclusion by chemical methods with tetra-substituted phosphonium/tetra-substituted borate (see Japanese Patent Application Laid-Open (JP-A) Nos. 49-118798, 9-328535 and 11-5829) and capsulation by physical methods with microcapsules (see JP-A Nos. 8-337633 and 9-77959) have been proposed, but none of these methods can satisfy both storage stability and rapid curing. Particularly, the microcapsules do not promise capsulation when the capsules are broken during production of resin compositions, while when the microcapsules are made so rigid as not be broken during production, the capsules are gradually destroyed in a curing reaction and are thus problematic in rapid curing. Under these circumstances, curing accelerators with priority on practically important rapid curing have been used thus requiring resin compositions to be stored, transported etc. at low temperatures.
In techniques of sealing elements of electrical parts, on the other hand, high-density mounting of electrical parts on a printed wiring board is advancing in recent years, and in consequence, surface-mounted packages are becoming predominant over conventional pin-inserted packages. However, the surface-mounted packages as compared with the pin-inserted packages tend to have lower resistance to package cracking upon soldering, that is, reflow crack resistance. That is, for increasing the package density of the surface-mounted package for IC, LSI or the like, the occupied volume of the element in the package is increased and the wall thickness of the package is made very thin. In addition, the surface-mounted package is subjected in a soldering step to severer conditions than the pin-inserted package.
More specifically, the pin-inserted package is not exposed directly to high temperatures because a pin is inserted into a wiring board and then the wiring board is soldered at the backside, while the surface-mounted package is exposed to high soldering temperature because the surface of a wiring board is subjected to temporary joining and then treated in a soldering bath or a reflow apparatus. As a result, when the package absorbs moisture, the absorbed water is rapidly expanded during soldering, thus leading to package cracking in some cases, which is a serious problem in package formation.
Under these circumstances, an epoxy resin composition with an increased content of an inorganic filler has been reported in order to improve reflow crack in the surface-mounted package. However, as the content of the inorganic filler is increased, the fluidity of the resin composition is decreased, so that at the time of formation, there often occur deteriorations in the performance of the package, for example, disturbances in formation such as insufficient filling and void generation or inferior conduction attributable to disconnection of bonding wires in IC chips. Accordingly, there is a limit to an increase in the content of the inorganic filler, and as a result, a significant improvement in reflow crack resistance is hardly achieved. Particularly when a phosphorus-based curing accelerator such as triphenyl phosphine or a nitrogen-containing curing accelerator such as 1,8-diazabicyclo[5.4.0]undecene-7 is added to such epoxy resin composition from the viewpoint of rapid curing, the resin composition tends to be significantly inferior in fluidity. Accordingly, there is demand at present for improvement in the fluidity of the resin composition in addition to improvement in the reflow crack resistance of the package.
For improving the fluidity of the epoxy resin composition containing an inorganic filler at a high ratio, a method of using, as a curing accelerator, an addition reaction product of triphenyl phosphine and 1,4-benzoquinone has been proposed (see JP-A No. 9-157497). As an alternative method, a method of using phosphoniophenolate as a curing accelerator has been proposed (see JP-A Nos. 2004-156035, 2004-156036 and 2004-176039).